Conventionally, an n-type or a p-type boron phosphide (BP)-based semiconductor layer has been employed for fabricating light-emitting diodes (LEDs) and laser diodes (LDs). For example, JP-A HEI 5-283744 discloses that a blue-LED is fabricated from a semiconductor structure including a silicon substrate, and an n-type BP layer to which silicon (Si) has been intentionally added and an aluminum gallium nitride (AlGaN) layer successively formed on the substrate. The prior art also discloses that a magnesium (Mg)-doped p-type BP layer is employed as a contact layer for fabricating an LED (see paragraph [0023] in the prior art).
As mentioned above, boron phosphide exhibiting a bandgap of 2.0 eV at room temperature is employed in combination with a Group III nitride semiconductor, such as AlXGaYIn1-X-YN (0≦X≦1, 0≦Y≦1), for fabricating a compound semiconductor light-emitting device (see, for example, JP-A HEI 2-288388). In the aforementioned LED emitting blue light of a wavelength corresponding to such a wide bandgap, a boron phosphide layer specifically serves as a base layer on which a Group III nitride semiconductor layer is grown, rather than as a cladding layer or a similar layer (see paragraph [0013] in JP-A HEI 5-283744).
In the case where a boron phosphide layer serving as a base layer is formed on a crystalline substrate, such as a silicon single-crystal substrate, it is known that the plane orientation of a surface of an epitaxially grown boron phosphide layer is determined in accordance with the crystal plane orientation of the surface of the substrate. For example, JP-A HEI 5-283744 discloses in paragraph [0025] that a (100) boron phosphide layer is grown on a (100) crystal plane of a silicon substrate and that a cubic AlGaInN layer is grown on the (100) crystal plane of the (100) boron phosphide layer. On the other hand, it is known that a (111) boron phosphide layer is grown on a (111) crystal plane of the silicon substrate and that a hexagonal AlGaInN layer is grown on the (111) crystal plane of the (111) boron phosphide layer.
The cubic AlGaInN, which is a promising candidate for a light-emitting layer or a similar layer, has a crystal structure less stable than that of a hexagonal Group III nitride semiconductor (see paragraph [0002] in JP-A HEI 5-283744). Thus, the cubic semiconductor cannot be formed in a stable state as compared with a hexagonal Group III nitride semiconductor, which is problematic.
As mentioned above, efforts have been made for growing a hexagonal AlGaInN layer having a more stable crystal structure on the (111) crystal plane of the boron phosphide layer formed on the (111) crystal plane of the silicon substrate. However, a portion in the hexagonal crystalline layer containing no cubic crystals is formed only in a limited portion from the junction interface with the boron phosphide base layer to the thickness less than 50 nm (see paragraph [0025] in JP-A HEI 5-283744).
In other words, even though it is intended that a hexagonal Group III nitride semiconductor layer is formed in a sufficient thickness on a boron phosphide-based semiconductor layer having a (111) silicon substrate, actual formation of the semiconductor layer is problematically difficult.
The present invention has been accomplished in view of the foregoing. Thus, an object of the present invention is to provide a boron phosphide-based semiconductor light-emitting device in which a high-crystallinity, hexagonal Group III semiconductor layer is formed in a sufficient thickness on a boron phosphide-based semiconductor layer provided on a silicon substrate, leading to manifestation of high emission intensity.